1. Field of the Invention
The present invention relates generally to a fluorescent lamp having a curved discharge path, such as a U-shaped path or a W-shaped path, and more particularly to a fluorescent lamp having a plurality of bulbs, the ends of which are connected by a cap, wherein the bulbs and the cap form a U-shaped, S-shaped, or W-shaped discharge path.
2. Description of the Related Art
A fluorescent lamp is used not only as a general room light source, but also as a special light source for various purposes.
For instance, a fluorescent lamp is employed as a "back light" for a liquid crystal display, which illuminates the rear surface of the display. In addition, the lamp is used as a light source in a copying machine, or other special light sources.
When the fluorescent lamp is employed as the back light for the liquid crystal display, it is necessary that the lamp illuminate the rear surface of the display almost uniformly. For this end, a number of straight-bulb fluorescent lamps are arranged in parallel, or a fluorescent lamp having a U-shaped or W-shaped bulb is employed.
When many straight-bulb fluorescent lamps are juxtaposed, many sockets and lamp holders are needed, and complicated wiring is required.
Under the circumstances, fluorescent lamps having U-shaped, S-shaped, or W-shaped bulbs are widely used. In this case, with use of a single lamp, a wide area can be uniformly illuminated.
Although the use of this single fluorescent lamp with a curved bulb reduces the number of sockets and simplifies the wiring, it is difficult to manufacture a bulb of such a complex shape.
On the other hand, there is known a so-called aperture-type fluorescent lamp which may be used as a back light for a liquid-crystal display. When a fluorescent lamp is used as a back light, it is not necessary that the bulb of the lamp emit light from its entire peripheral area. It suffices if light is emitted from only a given portion of its periphery. Taking this into consideration, the aperture-type fluorescent lamp has a slit-like aperture (opening) extending continuously in the axial direction of the bulb of the lamp. Phosphor is not coated on that part of the bulb, which corresponds to the aperture. A reflective coating layer is formed on that part of the outer surface of the bulb, which corresponds to the remaining inner surface of the bulb that is coated with phosphor. In the aperture-type fluorescent lamp, light emitted from the phosphor-coated part is reflected by the reflective coating layer, and most of the light is radiated through the aperture only in a predetermined circumferential range. Thus, the aperture-type fluorescent lamp can efficiently emit light only onto an area requiring light illumination.
An aperture-type fluorescent lamp having a curved bulb is manufactured in the following manner. First, a straight bulb is prepared. The inner surface of the bulb, excepting the area of a slit-like aperture, is coated with phosphor, and a reflective layer is coated on the outer surface of the bulb which corresponds to the phosphor-coated inner surface. The finished bulb is heated and partly bent in a U-shape or a W-shape.
When the aperture-type fluorescent lamp is employed as a back light for a liquid-crystal display, it is necessary, in some cases, to vary the axial length of the aperture or to change the position of the aperture in the circumferential direction of the bulb, in order to attain uniform illumination. In the case of the fluorescent lamp having the curved bulb, however, it is difficult to vary the length of the aperture and/or change the circumferential position of the aperture over the entire axis of the bulb.
When a plurality of fluorescent lamps having straight bulbs are arranged in parallel, as has been mentioned above, it is possible to turn on only some of them to change the amount of light and/or luminous intensity distribution. In the case of the fluorescent lamp having the curved bulb, however, it is not possible to turn on only a portion the bulb.
In the case where a fluorescent lamp is used as a light source in a copying machine, it is necessary, in some cases, to vary the wavelength of light in accordance with the color tone, etc. of an original to be copied. In a light source device wherein a plurality of fluorescent lamps are arranged in parallel, it is possible to arrange fluorescent lamps capable of emitting light of various wavelengths and to selectively turn on them to change the wavelength of emitted light. This is impossible, however, in the case of the fluorescent lamp having the curved bulb.
In addition, in the case of the fluorescent lamp with the curved bulb, a positive column generated in the bulb departs from the center of the cross section of the bulb towards the inner side of the curved portion. As a result, non-uniform light is emitted from the curved portion of the bulb.